1. Field of the Invention
This invention relates to a die clamping unit of an injection molding machine or the like, in which a molded piece is molded by injecting a material into injection molding dies, and a die clamping method, and more particularly, to a die clamping unit and a die clamping method that ensure shortened operating time.
2. Description of the Related Art
A die clamping unit of an injection molding machine or the like, in which a molded piece is molded by injecting a material into injection molding dies, comprises a stationary platen and a movable platen. The stationary platen is fitted with a stationary die. The movable platen is fitted with a movable die. The movable platen is attached to the stationary platen by tie bars. The movable platen can advance toward and recede from the stationary die. An injection molding machine of this type is described in Jpn. Pat. Appln. KOKAI Publication No. 10-296739, for example.
A die clamping unit described in Jpn. Pat. Appln. KOKAI Publication No. 10-296739 further comprises hydraulic die clamping cylinders and tie bar sliding servomotors (halfnut positioning servomotors according to this invention). The hydraulic cylinders are provided on the stationary platen and advance and retreat the tie bars. The tie bar sliding servomotors advance and retreat the tie bars in their axial direction. The movable platen is advanced and retreated on the tie bars by die opening/closing servomotors, and is anchored to the tie bars in a given position by die clamping halfnuts. In an injection pressing process, the hydraulic die clamping cylinders are driven to press the stationary and movable dies together.
Further, a die clamping unit having a die clamping ram is described in Jpn. Pat. Appln. KOKAI Publication No. 6-182838. This die clamping unit comprises a stationary platen, movable platen, die clamping ram, and hydraulic die clamping cylinder. A stationary platen is fitted with a stationary die. The movable platen is fitted with a movable die. The movable platen is attached to the stationary platen by tie bars. The movable platen can advance toward and recede from the stationary die.
The die clamping ram has a fitting portion that projects in its central part and is attached to the movable platen. The fitting portion of the die clamping ram is driven by the hydraulic die clamping cylinder for advance and retreat. In the injection pressing process, an oil pressure is fed into the hydraulic cylinder so that the movable platen is moved by the die clamping ram. By this operation, the stationary and movable dies are pressed together.
A die clamping unit described in Jpn. Pat. Appln. KOKAI Publication No. 2002-127216 further comprises a die clamping mechanism. This mechanism connects and pressurizes a movable platen plate (movable platen according to this invention) and a stationary platen plate (stationary platen according to this invention) by means of tie bars. The die clamping mechanism comprises a two-stage die clamping cylinder, which is composed of large- and small-diameter cylinders, and large- and small-diameter rams. A suitable oil pressure is applied to the large- and small-diameter cylinders to press the stationary and movable dies together. The small-diameter cylinder is operated to remove clearances between sawtooth die clamping halfnuts and sawtooth screw portions of the tie bars. Subsequently, an oil pressure is applied to the large-diameter cylinder to carry out die clamping operation.
The die clamping unit described in Jpn. Pat. Appln. KOKAI Publication No. 10-296739 or No. 6-182838 operates in the following manner. First, the tie bars are advanced or retreated by the tie bar sliding servomotors, and the respective phases of the sawtooth die clamping halfnuts and the sawtooth screw portions of the tie bars are aligned. Subsequently, the die clamping halfnuts and the screw portions are engaged with one another. Thereafter, the hydraulic die clamping cylinders are driven to press the stationary and movable dies together.
In this case, clearances are provided between sawteeth to allow the die clamping halfnuts and the screw portions of the tie bars to smoothly or easily engage one another. When generating a die clamping force, the clearances that result after the halfnuts and the screw portions are engaged need to be removed. More specifically, oil pressure is applied to the hydraulic die clamping cylinders such that a force is transmitted to the halfnuts through the tie bars to remove the clearances between the sawteeth. The removal of the clearances between the sawteeth of the die clamping halfnuts and the screw portions of the tie bars can take a considerable amount of time. Thereafter, the die clamping force is transmitted to the movable platen. Thus, there is a problem that much time is typically required from the start of operation until the die clamping operation because to the time it takes to remove the clearances between sawteeth.
The capacity of the hydraulic die clamping cylinders must be increased in order to obtain a large die clamping force that would quickly remove the clearances between the sawteeth of the die clamping halfnuts and the screw portions of the tie bars. If the cylinder capacity is large, however, a large amount of oil must be supplied to produce the necessary die clamping force and large-sized hydraulic pumps would be needed to provide the oil to the hydraulic die clamping cylinders. Conventional composite die clamping units generally use a lot of hydraulic pumps and feeding a large discharge of oil pressure into the hydraulic cylinders to quickly remove the clearances between the sawteeth is typically not a problem.
For composite die clamping units of hybrid injection molding machines that are currently used, however, the discharge of the hydraulic pumps is is less than in, for example, conventional units, and the motors are miniaturized to limit or reduce power consumption. Thus, in these types of injection molding machines, it takes a lot of time to remove the clearances between the sawteeth.
Like the die clamping unit described in Jpn. Pat. Appln. KOKAI Publication No. 2002-127216, moreover, some die clamping units are provided with a small-diameter cylinder of small capacity besides a large-diameter cylinder. In these die clamping units, the operating time is shortened by performing the operation to remove the clearances between the sawteeth with the small-diameter cylinder. Since the large-diameter cylinder is subjected to the oil pressure after the small-diameter cylinder, however, the application of the oil pressure requires switching operation. Thus, applying the oil pressure is time-consuming, so that the operating time cannot be shortened.